Another Dimension: Or at least a "new universe", if Erik Selvig's comment in The Avengers is anything to go by.
Arm Cannon: The cannons that some of the Chitauri have (including the eponymous Item 47) seem to be fit around their arms to fire one-handed. They, however, do not seem to actually be the arm, since an arm falls out when Captain America chops it off of one of them.
Awesome Personnel Carrier: Their main transport is the Leviathan, a gigantic flying serpent-like creature that carries hundreds of soldiers and can fly through skyscrapers. It takes huge amounts of explosive or electrical damage to put one down. Or one Hulk punch and a several Stark missiles.
Or blow one up from inside.
Bayonet Ya: Their infantry sport them on their rifles.
Blood Knight: They grow restless when denied war, to the point where they'll attack each other. The tactical problems with this are summed up in a deleted scene:
Expy: Of Darkseid's Parademons. Which is fitting, since their leader, Thanos, was originally designed as a Darkseid expy.
Frickin' Laser Beams: Two varieties. One is fired from a thin spear/rifle used by Chitauri foot soldiers. The other is a powerful Arm Cannon that's mounted on Chitauri commanders and units riding in hovercraft. Both fire blue colored beams.
Humanoid Aliens: Barring the freaky double-thumbs and evil-looking skull-faces.
Humanoid Aliens: He has the same general shape as a human (and of course is played by a human actor), but the details are very different. For starters, he has two thumbs on each hand. It's not really clear if he's of the same race as the Chitauri.
In the Hood: His hood helps to further obscure his face.
Smug Snake: He's certain that Earth will surrender the instant his Chitauri launch their assault (see his quote). He wisely becomes more cautious of the humans after the Avengers repel the invasion. This might qualify him as a subversion, as a key characteristic of a Smug Snake is an inability to learn from their mistakes, a weakness he clearly does not share.
Teeth-Clenched Teamwork: From the start, he shows annoyance over working with Loki. Though, it was ultimately up to Thanos to make that decision, and gave him both his army as well as the scepter.
Bigger Bad: Definitely the most powerful villain yet seen in the MCU, and anyone familiar with his comic counterpart knows he's certainly evil, but he's not the major antagonist of any of films yet and, until The Avengers, not the direct cause of any of the conflicts of them. He's just lurking in the background. The existence of the Infinity Stones/Gems in the films all but confirms he'll step into his own as a main Big Bad eventually.
As Himself: Except perhaps in Iron Man 1 and 2, where he may or may not have been Hugh Hefner and Larry King. Definitely not himself in Captain America, since he would been about 60 years younger at that point in time.
Butt Monkey: Mistakes a Senator's aide for Captain America, Tony Stark can never get his name right, gets sick from a gamma radiation laced soft drink, has the back of his truck ripped off, and misses all of the Avengers' action in New York. By Thor: The Dark World, he's in an asylum. In Captain America: The Winter Soldier, he's a Smithsonian guard who's going to lose his job after Steve breaks in and takes his WWII uniform to wear in the climax of the film.
Creator Cameo: With the exception of his appearance in Captain America. He did not create that character but was responsible for bringing him into the Silver Age, as well as creating Cap's now-iconic shield throwing.
One half of the Outlaw Couple that formed following the Battle of New York. After getting pitched the crazy idea to rob banks with his girlfriend, he went along with it reluctantly, but deep down is grateful for the chance to finally live out their dreams.
Anti-Villain: Just an ordinary guy reluctantly robbing banks to fix his life's problems.
Boxed Crook/Cut Lex Luthor a Check: At the end of Item 47, he's given a job at SHIELD in their Research and Development department's "Think Tank" due to his skill with Chitauri weaponry.
Gadgeteer Genius: He manages to make the Chitauri gun work, while SHIELD hasn't made any progress with it.
Claire's had a hard life of bills, rejected dreams, and alien invasions. But when a Chitauri gun literally falls out of the sky as if as a present, she sees the chance to finally take matters into their own hands and gain the means to live like they want to.
Anti-Villain: Just an average woman with bills to pay who thinks she could solve her problems with bank robbing.
The Mandarin is the mysterious leader of the Ten Rings terrorist organization that menaced Tony in the first Iron Man film. Though Aldrich Killian's scheme to use the Mandarin's image made it seem that he was a mere fabrication, the Marvel short All Hail the King reveals that he is indeed real. He orders his subordinate Jackson Norris to break Trevor Slattery, his impostor, out of prison so he may punish him personally.
Bigger Bad: He's behind the Ten Rings, but hasn't yet stepped into the spotlight with any evil plans.
The Ghost: This character has yet to actually appear.